Emergency lighting systems provide necessary light and protect against the dangerous conditions that may exist due to the lack of adequate lighting during a lighting emergency. Such a lighting emergency may occur when there is an interference with the normal electricity provided in a building or to the building's surroundings. In lighting emergencies it is often beneficial to illuminate specific areas, signs, exit signals, walkways, stairways, or to otherwise allow for the use of and exit from the premises by providing light or signals. Emergency lighting systems detect a lighting emergency and allow for automatic and adequate illumination when such an emergency occurs.
An emergency lighting system is one or more device, unit, apparatus, equipment or component used to detect the existence of a lighting emergency and/or provide emergency lighting or power for emergency lighting in a lighting emergency. An emergency lighting system may include one or more separately placed emergency detection and emergency lighting units.
Some emergency lighting systems detect the existence of an emergency condition by monitoring the electricity supplied to a building, or a portion of a building, and detecting when the electrical conditions are inadequate to provide for normal lighting. Note that the use of emergency lighting systems is not restricted to buildings. Emergency lighting is generally applicable anywhere that lighting or electricity is used. In that respect, the technology surrounding emergency lighting systems has far reaching applications, including potential uses in electronic devices, computers, automobile electronics, aircraft electronics, and many others.
Many emergency lighting systems are capable of detecting electrical conditions that are inadequate to provide for normal lighting. One such condition, known as a blackout, occurs a when a building, portion of a building, or other area loses power. Generally, such a condition occurs when the external power being supplied, is substantially or completely shut off, when there is a power outage, power failure, or other significant power disturbance. During a blackout, an emergency lighting system may detect and respond to the emergency condition by illuminating emergency lights and signs.
A brownout is another type of emergency lighting condition. A brownout occurs when there is a reduction of normal voltage in the electricity provided to a building, or portion of a building. For example, a brownout may occur on a summer day when the demand for electricity from a given power company is particularly high causing a drop in the voltage level provided by that power company. The voltage level may drop to the point where some or all of the normal lighting circuits cease to function.
Some emergency lighting systems utilize brownout detection circuitry to detect a reduction in the voltage that is supplied to the normal lighting system. The brownout detection circuitry is generally intended to recognize a situation when the voltage is too low for normal lighting circuits to provide illumination for a space. Note that some brownout detection circuitry may also detect a blackout since during a blackout there is little or no voltage or power. In many systems, once the brownout detection circuitry makes a determination that the voltage is too low, a battery powered lamp is turned on to provide lighting until the voltage rises to a point sufficient to resume providing illumination through the normal lighting system. The emergency lamps are usually direct current (“DC”) powered lamps intended to efficiently provide an adequate level of illumination to ensure safety during the power emergency.
A building's normal electricity supply provides an input voltage that may be both monitored and used by an emergency lighting system. Emergency lighting systems use the input voltage, usually after converting the voltage to direct current, to provide power to charge or recharge a battery. The systems also monitor the input voltage for emergency lighting condition such as a blackout or a brownout. Accordingly, many emergency lighting systems include (1) powering circuitry to use the input voltage to power the charger and (2) detection circuitry to monitor the input voltage for the existence of an emergency condition.
Prior art methods of powering the charger and detection circuitry in emergency lighting systems typically have involved either a transformer or capacitive input circuit. These components were used to change the voltage level or otherwise alter the electricity provided by the input voltage to an appropriate type useful to the battery charger or detection circuitry.
A transformer is an energy coupling device that takes electrical energy at one voltage and transforms it to another voltage. The new voltage may be higher (stepped up) or lower (stepped down), or it may remain the same as the input voltage. For example, if an input voltage of 480V, the rated voltage, is applied to the primary of a 480V-120V single winding transformer, the secondary voltage produced by the transformer will be 120V. In use, however, the input voltages are often higher or lower than the rated voltage of a transformer's primary. In these instances the secondary voltage will be higher or lower respectively. For example, a 480-120V single winding transformer with an input line voltage of 456V will have a secondary output voltage of 114V. This is because the transformers voltage ratio is 4:1 (480V primary divided by 120V secondary). Thus, its secondary voltage is 456V divided by 4, or 114V. Conversely, this same transformer with an input voltage of 504V will have a secondary voltage of 126V (504V divided by 4).
Transformers often have one or more voltage taps. A voltage tap is an additional connection on either the primary or secondary side of the transformer. A voltage tap allows the user of the transformer to alter the transformer's voltage ratio. As described above, the voltage ratio determines the voltage transformation that takes place. There are times when the actual incoming voltage is different than the expected normal incoming voltage. When this happens, it may be advantageous to be able to change the voltage ratio in order to get the desired (rated) output voltage. Voltage taps, designed into the transformer's primary, deliver this desired flexibility. In other words, tapping the primary in a number of different spots provides a means to adjust the turns ratio and fine-tune the secondary output voltage. These tap connections are usually set at the factory for normal line voltages. During installation, the appropriate tap may be selected depending on the input voltage present at the installation site.
In emergency lighting systems, transformers have been used to step down an input voltage to a lower voltage, which is then used to power the charger circuitry. Because the transformer could have multiple input voltage taps, the transformer could accept input voltages of various magnitudes allowing the emergency lighting system to be used in different voltage environments. For example, one common method has been to utilize a 60 Hz line rated transformer with taps for 120 and 277 VAC. During installation the electrician could select the appropriate tap for the voltage level at the site.
Capacitive divider circuits are also used to step down an input voltage to power the charger circuitry in emergency lighting systems. Like transformers, capacitive divider circuits can also have taps, which allow the use of emergency lighting systems using these circuits in different voltage environments. For example, a capacitive divider circuit with taps for 120 and 277 VAC could be used in an emergency lighting system.
The use of a transformer or capacitive divider circuit in past emergency lighting systems allowed for relatively simple brownout detection circuitry in those systems. There were two main categories of brownout detection circuits used in these systems. Some brownout circuits utilized a set input voltage tap on the primary side of the transformer or capacitive divider while other brownout detection circuits used the secondary voltage produced on the secondary side of the transformer or capacitive divider.
In the first category of brownout detection circuits mentioned above, the circuit took advantage of the availability of a set voltage tap, usually 120V, present on in the emergency lighting system. The above methods of powering the charger circuitry with a transformer of capacitive divider circuit insured that no matter what voltage was applied to the system in the field, a set voltage tap, usually 120V, of the transformer or capacitive divider always had a set voltage present that varied proportionally with the input voltage. Because this was the case, a simple circuit could be used to generate a DC voltage that was the same regardless of the input tap selected and in proportion to the incoming AC input voltage regardless of the input voltage level. The ability to generate a single DC voltage proportional to the input voltage level regardless of the input voltage level meant that a simple comparator circuit could be used to detect a brownout condition by detecting drops in the input voltage.
In the second category of brownout detection circuits mentioned above, the circuit utilized the secondary voltage produced on the secondary side of the transformer or capacitive divider. These circuits use a reduction in the voltage on the secondary side of the transformer or capacitive divider to infer a reduction in the input voltage on the primary side.
Although detecting brownout conditions on the secondary side works reasonably well, this method has significant disadvantages. Specifically, the loading of the charger circuitry by a discharged battery can be mistaken for a reduction in the input voltage. Because of this disadvantage, circuits that employ this method for detecting a brownout condition normally have the point where the brownout circuit turns on the emergency circuit, the brownout threshold, preset at a significantly lower percentage of normal input voltage (eg. 65-70%) to avoid false triggers of the brownout circuit under conditions of heavy transformer loading. In other types of brownout circuits the brownout threshold would typically be preset at around 80% of the nominal input voltage. Because the brownout threshold is set lower for brownout detection circuits that use the secondary side, these circuits increase the likelihood that input conditions may exist where the normal lighting circuits have failed but the emergency lighting has not started to provide illumination.
As switch mode power converter technology has been improved significantly over the past several years, it has now become economically feasible to replace the common transformers or capacitive divider circuits used extensively in the past in emergency lighting systems with a switch mode power converter. A switch mode power converter is an example of a wide input supply range converter. This type of circuitry offers the advantage of being able to operate over a wide input supply range (85-305 VAC 50-60 Hz) that eliminates the voltage specific taps needed with transformer or capacitive input circuits. The flexibility of this type of input circuitry offers many advantages over using transformers or capacitive dividers with either type of brownout detection described above. First, a switch mode power converter provides greater input range flexibility. Odd voltage and frequency (eg. 220V 50 Hz) AC input requirements can be met without having to specify different transformer types or capacitor values. Second, there is a reduction in the likelihood of field wiring mistakes that can occur when an electrician selects the wrong tap to power the system. Third, using a switch mode power converter instead of a transformer or capacitive divider circuit may allow the size of the emergency lighting system to be reduced.
The conventional methods of brownout detection cannot be used when a switch mode power converter is used in place of a transformer or capacitive divider circuitry. This is the case because using the switch mode power converter circuitry eliminates key elements normally relied upon to implement a simple, low cost, brownout detection circuit.
A first problem is that the set voltage tap is not available because of the wide input voltage range topology inherent in the switch mode power converter. Without this tap there is no common reference point that can be used to generate the single voltage level that is proportional to the input voltage regardless of the value of that input voltage. Accordingly, brownout detection circuits that rely on this set voltage tap cannot be used with a switch mode power converter.
Another problem is that the change in the secondary output voltage over the wide range of input voltages is tightly regulated and therefore not useful to brownout detection circuits. This means that brownout detection topologies that rely on secondary side outputs can no longer be used to determine the change in the input voltage on the primary side of the circuit in systems that utilize a switch mode power converter.
Another complication in implementing common brownout detection techniques with switch mode power converter technology results from the frequent requirement in emergency lighting equipment to isolate the primary and secondary sides of the circuit. Specifically, the brownout detection circuitry located on the primary side of the circuit must communicate or send its output to the circuitry that controls the lamps or lighting on the isolated secondary side of the circuit.